The overall aim of the proposed research is to further analyze serologically the chromosome region in man which includes and surrounds the major histocompatibility locus (HL-A). The program will be directed 1) to extending the definition of the HL-A locus and 2) to detecting the lymphocyte defined (LD) non-HL-A loci by serologic means. A search will be instituted for human antisera which identify 1) new alleles at the HL-loci and 2) alleles governing the I region (MLR, Ir, GVHR) (MLR-mixed lymphocyte culture reaction, Ir-immune response, and GVHR-graft versus host reactions)1expressed on the surfaces of thymus derived lymphocytes (T cells) and/or bone marrow derived lymphocytes (B cells). The interaction of the various serologically defined antigen systems will be correlated with disease susceptibility, with immune responses to various antigens and with the outcome of organ transplantation. BIBLIOGRAPHIC REFERENCES: Payne, R., Radvany, R., and Grumet, C.: A New Second Locus HL-A Antigen in Linkage Disequilibrium with HL-A2 in Cantonese Chinese, Tissue Antigens 5: 69, 1975; Grumet, C., Payne, R., Konishi, J., and Kriss, J: HL-A Antigens as Markers for Disease Susceptibility and Autoimmunity in Graves' Disease, J. Clin. Endocrin. and Metab. 39: 1115, 1974.